This invention generally relates to an improved method of preparing an aluminum surface for the application thereto of curable organic matter, such as paints or adhesives.
Over the years, many processes have been described for pretreating an aluminum surface in preparation for subsequent painting, adhesive bonding and the like. Most of these processes fall into one of two classifications, namely, chemical conversion coating or anodizing. Although anodized coatings can be excellent bases for paints and adhesives, generally, they are two expensive for most commercial applications. As a result, only chemical conversion coatings have been used to any significant extent in commercial practice, particularly in continuous coil coating operations. Chemical conversion coating processes for aluminum usually involve utilization of a highly acidic solution containing one or more species of chromates (primarily dichromates) frequently fluorides and/or phosphates and various types of accelerators such as ferricyanides. Conventional chemical conversion coating solutions usually contain proprietary products, such as Alodine 1200, Alodine 401-45, Bonderite 78 and the like. These commercial products form coatings which are excellent bases for paints and adhesives. For example, painted aluminum products with a conventional chemical conversion coating base have been subjected up to twenty years or more of outdoor exposure with no serious detrimental effects.
Increased concern of late about environmental pollution has resulted in legislation which severely restricts what chemicals can be discarded into waterways and local sewage treatment facilities. Such restrictions now usually require the complete treatment of spent chemical conversion coating solutions to remove therefrom the undesirable chromates, fluorides, phosphates, cyanides and the like prior to discharge. Treatment of the spent solutions usually include passing the solution through ion exchange resins in addition to other treatments which increase considerably the cost and complexity of chemical conversion coating processes. This is particularly evident in continuous coil coating operations where large volumes of chemicals are employed.
Efforts have been made to reduce the amount of chromates, fluorides and phosphates in the chemical conversion coating solutions, but the quality of resultant painted products is frequently less than satisfactory. In U.S. Pat. No. 3,488,228 and U.S. Pat. No. 3,531,332, a process is described wherein an aluminum surface is treated with an aqueous solution of certain amines which generate a pH between 7 and 11. Improved adhesion between the aluminum substrate and organic coatings, such as paint or adhesive, is alleged. One of the stated advantages is that no chromates or fluorides are used in the treating solution. However, to date no significant commercial use has occurred of such conversion coatings or any other coatings employing no chromates, fluorides or phosphates as a paint or an adhesive base has occurred to date.
It is against this background that the present invention was developed.